The sealing of joints in a composite aircraft made from epoxy resins presents two problems with respect to the seals, particularly with respect to fuel tanks which would be positioned integrally in the aircraft, primarily in the wings. The first is an electrical current conduction problem in the composite joints and the second is in the repair of such joints. The possibility of repairs can relate to any part of the aircraft but particularly are required in fuel leak problems in the integral fuel tanks. Fuel leaks, particularly in military aircraft, are very common and are a high maintenance cost item. It is expected that this problem will exist in the same manner and to the same extent in composite aircraft where the fuel tanks are made of graphite reinforced epoxy resins.
None of the presently used sealing concepts in fuel tanks, except one, can stand up to the rigors of high loading and temperature extremes over an extended period of time. This fuel sealing concept with unusual reliability is in a bonded/bolted concept known as the "Scotchweld Process". It is usable in aluminum structures but is not applicable to composite structures because of the repair procedure. The repair procedure requires that the structure be cooled below freezing and that a wedge be driven between the aircraft skin and the substructure to break the adhesive bond that forms part of the tank seal. This repair operation does not damage the aluminum structure but if it were attempted with composite structures formed of epozy resin plates, the skins and substructure of the plates would delaminate because the presently used matrix epoxy materials are more brittle than the adhesives, especially at low temperatures.
A search of the patent literature was made with respect to the electrical current conduction problem and the fuel tank leak problem but no solutions were found. For example, a Netherlands publication No. 7,404,508, dated Oct. 4, 1974, disclosed the embedding of particles in a thermoplastic layer bonded to metal. These bonds were provided to produce peel resistance of the plastic so as to cause it to separate from the metal along the line where the embedded particles would lie when the plastic is pulled in tension at right angles to the metal surface. When pulled at right angles, the plastic breaks along the embedded particles.
U.S. Pat. No. 3,347,978 suggests the use of carbon particles in a resin for rendering adhesive joints electrically conductive, column 3, line 62.
U.S. Pat. No. 3,728,427 teaches the use of a static electricity bleed in the form of a resinous gel which contains conductive metal slivers, flakes or needles.
The following patents found are considered to be of general interest:
U.S. Pat. No. 3,140,342; U.S. Pat. No. 3,726,738; U.S. Pat. No. 3,707,429; U.S. Pat. No. 3,795,047; and British Pat. No. 698,210/53.